The art of injection molding is well known and documented, but some aspects of producing the perfect molded product is elusive. The basic process of melting, or plasticizing, a resin in an injection molding machine, then conveying said molten resin under pressure through melt channels in a heated manifold system to mating nozzle housings is proven. Each heated nozzle housing melt channel is in fluid communication with respective manifold melt channels to facilitate conduction of the resin at a uniform temperature throughout the hot runner, culminating in passage through a nozzle tip, distally attached to the nozzle housing, through a gate orifice and finally in deposition within a mold cavity, to take the shape of the molded product.
While the resin is optimally heated beyond its melting point throughout its journey within the hot runner, from sprue bushing to nozzle tip, in order to reduce the injection pressure required to move it, the resin must then be quickly cooled once it reaches the gate orifice and mold cavity to facilitate its formation into the final molded product. Both the tip surrounding gate insert as well as the mold cavity and mold core are cooled to a temperature below the melting point of the resin to hasten its solidification. Once the molten resin is injected into the mold cavity, it is afforded a predetermined amount of time to cool down below its softening temperature, so that it will retain its shape and break away from the gate orifice cleanly, when the mold cavity separates from the mold core and the molded product is ejected to further downstream product handling apparatus.
The two most popular methods of gating a molded product, or controlling the flow of resin through a small gate orifice adjacent the mold cavity, are using a valve gate or a hot tip type nozzle assembly. A valve gate device regulates the flow of molten resin through the gate to the mold, while affording precise control of the process, typically via a pneumatically or hydraulically controlled valve stem which reciprocates axially through the center of a nozzle housing melt channel. When cycled back and forth, the valve stem either retracts from the gate orifice to allow resin flow, or intimately mates with the gate orifice to mechanically shut off and prevent resin flow, through the gate orifice. While this type of mechanical control of resin flow through the gate is relatively accurate, its drawbacks include requiring significant space within the hot runner plates to enclose the numerous valve stem drive mechanisms, such as pistons and pneumatic or hydraulic cylinders, which translates to increased costs for thicker hot runner plate steel, the additional cost of a valve stem, piston and the means to drive them for each mold cavity and the cost of replacing or reworking the mating valve stem and gate inserts due to wear and abrasion accumulated from repeated contact during each molding cycle.
Comparatively, the hot tip method of gating to a molded product is a less cumbersome and less expensive device involving simply a heated nozzle housing to convey pressurized resin flow toward a thermally conductive nozzle tip having outlet ports to allow egress of the molten resin through to the adjacent gate orifice. Since this method of transmitting molten resin affords no mechanical means of control of its behaviour at the gate orifice, certain drawbacks of this nozzle tip style are also well known to those skilled in the art of injection molding.
At the beginning of an injection cycle, molten resin is forced, under high speed and pressure, through the hot runner culminating at the nozzle tip where, due to the decreasing orifice diameters, a high shear rate is encountered by the resin which adds additional shear heating to the local area of the nozzle tip and the gate orifice. These conditions help to force the cold slug of resin initially occupying the gate area through the gate orifice and re-melt the cooler layers of resin built up along the gate bubble to allow the flow of new resin into the empty mold cavity. Once the mold cavity is filled with resin, it is held in place for a period of time so that it may cool sufficiently. A layer of static resin solidifies, or freezes, in the gate bubble during this hold time and this solidified resin, or cold slug anchors the molded part to the gate insert. After sufficient cooling time has elapsed, the mold opens and the molded product separates from the gate orifice such that it takes with it a fractured, frozen layer of resin and leaves behind a cold slug in the gate area which acts as a sealing diaphragm, and the cycle continues.
Inherently, the hot tip gating method produces undesirable effects at the gate orifice such as a ‘string’ and a ‘vestige’. A ‘string’ is defined as thin, fine hair-like extension of warm resin emanating from the gate orifice, attached to the molded product, which upon part ejection from the mold, usually necessitates downstream handling to trim it from said molded product. A string is caused by the remaining sealing diaphragm being too thin, or being too high in temperature, as well as a high residual pressure within the hot runner.
A ‘vestige’ is defined as a raised witness mark, or post, which is left on the molded product at the location of the gate orifice, and is caused by a lack of a definitive breaking point of the resin attached to the molded product at the gate orifice often caused by either the nozzle tip being recessed too far back from the molding surface or a gate orifice diameter which is relatively too large, or a combination of both. A vestige is characterized by the shape of the frozen layer left on the molded product which defines the resulting gate vestige quality.
The most favorable gate vestige left on a molded product is achieved by rapidly solidifying the resin in the gate area to form an optimal sealing slug before the mold breaks open. The factors affecting this outcome include nozzle tip geometry, gate bubble geometry, gate cooling efficiency and each of their associated temperatures during processing. Additionally, resin processing conditions are factors, specifically the solidification properties of the resin, the residual pressure in the hot runner as well as the amount of time the molded part is held and allowed to cool before ejection from the mold.
Upon mold opening it is desired to generate a brittle fracture across the solidified resin or cold slug such that there is left behind a solid sealing diaphragm in the gate orifice area. Variable mechanical influences, including gate notch geometry and surface finish, as well as resin behaviour and its processing conditions, such as the resin's slug geometry, temperature, fracture properties, and notch sensitivity, and mold break speed, all contribute to forming the optimal gate.
Current hot tip designs provide heat to the gate area to keep the resin molten to prevent its premature cooling or ‘freezing’ to a temperature below the melting point of the resin during injection. The problem with these designs is that the molten layer created around the tip surface extends beyond the gate orifice and into the mold cavity such that when the mold opens, following a holding stage at which the molded product is allowed to set, it breaks both the solidified and molten resin layers which results in a string. Ideally, to eliminate the string, only the cooled and solidified resin layer should be broken at part ejection. One solution to achieve this is to recess the nozzle tip from the mold cavity surface essentially cooling the gate area by virtue of removing the heat source that is the thermally conductive nozzle tip. While this proposed solution may reduce the frequency of stringing it comes at the expense of creating the potential of high gate vestige. Another solution to minimize potential stringing is to hold the molded part in the mold for an extended period of time so that the cooled mold cavity and core can extract sufficient heat from resin in the gate area thus rendering a more solidified resin. Unfortunately, this method negatively impacts the overall cycle time to produce each molded part and so reduces the potential quantities created per unit time, ultimately impacting realized revenue by the molder.
Still other hot tip designs utilize a thermally conductive tip body fitted with a tip insert made of carbide which, due to its high hardness, is used for its abrasion resistance. While this design prolongs the life of the tip insert by slowing its wear due to erosion from the resin flow, the carbide material has a relatively high thermal conductivity and as such, does little to prevent resin stringing at the gate area.
U.S. Pat. No. 6,164,954 to Mortazavi et al describes an inner portion of an injection molding device, a nozzle tip, as being made of, or having an inner layer or coating of, various materials, including ceramics. The objective of this design is to incur high wear resistance and excellent thermal conductivity. Notwithstanding that the nozzle tip design is for use with a valve gate tip, and so, has no apex to occupy the gate orifice, the conduction of heat to the gate area is undesirable due to the tendency to promote resin stringing.
U.S. Pat. No. 7,175,416 B2 to Baresich et al describes an injection molding nozzle of thermally conductive material which strives to reduce the potential for stringing by geometrically enhancing the nozzle tip such that the wall thickness of said nozzle tip is severely restricted at the tip end thereby choking off the conductive flow of heat within the tip. The resulting, relatively cooled, resin near the tip end is said to have fewer tendencies to produce strings. However, while the nozzle diameter is reduced, it is still an integral part of the nozzle body and, as such, is made of the same thermally conductive material. This limits the desired effect as heat will still be conducted to the tip end at the same rate on a molecular level. Also, because the resin flow of the nozzle in question is axial to the tip, it allows for the warmest resin to occupy the center of the gate orifice area, and since the resin is surrounded by the heated cylinder that is the nozzle tip, it acts as a barrier which precludes the cooled gate insert walls from efficiently removing heat from the central resin flow, which is critical to preventing stringing. Additionally, the cylindrical tip end geometry does not address the resin fracture mechanics and its effect on gate vestige on the molded product.
For the foregoing reasons, the present invention is directed to overcoming one or more of the problems or disadvantages set forth above, and for providing a thermally conductive nozzle tip body featuring an insulating, low thermal conductivity, tip insert which will allow resin to freeze more quickly and specifically at the gate orifice to prevent resin stringing and minimize gate vestige on the molded product.